Remove delete_recursively

Looks like this was copied from Lwt at some point. It is not used within
obuilder currently, nor have I found it used in any of our other
projects. And it has since been exposed in Lwt. See
https://github.com/ocsigen/lwt/blob/48abed72467ca7479e95f1be06d02f40c7c434bd/CHANGES#L25

lib/os.ml

@@ -232,8 +232,6 @@ let rm ~directory =
     Log.warn (fun f -> f "Failed to remove %s because %s" directory m);
     Lwt.return_unit
 
-(** delete_recursively code taken from Lwt. *)
-
 let win32_unlink fn =
   Lwt.catch
     (fun () -> Lwt_unix.unlink fn)
@@ -255,31 +253,6 @@ let unlink =
     win32_unlink
   else
     Lwt_unix.unlink
-
-(* This is likely VERY slow for directories with many files. That is probably
-   best addressed by switching to blocking calls run inside a worker thread,
-   i.e. with Lwt_preemptive. *)
-let rec delete_recursively directory =
-  Lwt_unix.files_of_directory directory
-  |> Lwt_stream.iter_s begin fun entry ->
-    if entry = Filename.current_dir_name ||
-       entry = Filename.parent_dir_name then
-      Lwt.return ()
-    else
-      let path = Filename.concat directory entry in
-      Lwt_unix.lstat path >>= fun {Lwt_unix.st_kind; _} ->
-      match st_kind with
-      | S_DIR -> delete_recursively path
-      | S_LNK when (Sys.win32 || Sys.cygwin) ->
-        Lwt_unix.stat path >>= fun {Lwt_unix.st_kind; _} ->
-        begin match st_kind with
-          | S_DIR -> Lwt_unix.rmdir path
-          | _ -> unlink path
-        end
-      | _ -> unlink path
-  end >>= fun () ->
-  Lwt_unix.rmdir directory
-
 let normalise_path root_dir =
   if Sys.win32 then
     let vol, _ = Fpath.(v root_dir |> split_volume) in